Ventricular arrhythmia incidence in the rat is reduced by naloxone.

This study characterized the antiarrhythmic effects of the opioid receptor antagonist naloxone in rats subject to electrically induced and ischemic arrhythmias. Naloxone (2, 8 and 32 µmol/kg/min) was examined on heart rate, blood pressure, and the electrocardiogram (EKG) as well as for effectiveness against arrhythmias produced by occlusion of the left anterior descending coronary artery or electrical stimulation of the left ventricle. Naloxone reduced blood pressure at the highest dose tested while heart rate was dose-dependently reduced. Naloxone dose-dependently prolonged the P-R and QRS intervals and increased the RSh amplitude indicative of effects on cardiac sodium (Na) channels. Naloxone prolonged the Q-T interval suggesting a delay in repolarization. Naloxone effects were comparable to the comparator quinidine. Naloxone (32 µmol/kg/min) reduced ventricular fibrillation (VF) incidence to 38% (from 100% in controls). This same dose significantly increased the threshold for induction of ventricular fibrillation (VFt), prolonged the effective refractory period (ERP) and reduced the maximal following frequency (MFF). The patterns of ECG changes, reduction in ischemic arrhythmia (VF) incidence and changes in electrically induced arrhythmia parameters at high doses of naloxone suggest that it directly blocks cardiac Na and potassium (K) ion channels.

The antiarrhythmic actions of bisaramil and penticainide result from mixed cardiac ion channel blockade.

Decades of focus on selective ion channel blockade has been dismissed as an effective approach to antiarrhythmic drug development. In that context many older antiarrhythmic drugs lacking ion channel selectivity may serve as tools to explore mixed ion channel blockade producing antiarrhythmic activity. This study investigated the non-clinical electrophysiological and antiarrhythmic actions of bisaramil and penticainide using in vitro and in vivo methods. In isolated cardiac myocytes both drugs directly block sodium currents with IC50 values of 13µM (bisaramil) and 60µM (penticainide). Both drugs reduced heart rate but prolonged the P-R, QRS and Q-T intervals of the ECG (due to sodium and potassium channel blockade) in intact rats. They reduced cardiac conduction velocity in isolated rat hearts, increased the threshold currents for capture and fibrillation (indices of sodium channel blockade) and reduced the maximum following frequency as well as prolonged the effective refractory period (indices of potassium channel blockade) of electrically stimulated rat hearts. Both drugs reduced ventricular arrhythmias and eliminated mortality due to VF in ischemic rat hearts. The index of cardiac electrophysiological balance (iCEB) did not change significantly over the dose range evaluated; however, different drug effects resulted when changes in BP and HR were considered. While bisaramil is a more potent sodium channel blocker compared to penticainide, both produce a spectrum of activity against ventricular arrhythmias due to mixed cardiac ion channel blockade. Antiarrhythmic drugs exhibiting mixed ion channel blockade may serve as tools for development of safer mixed ion channel blocking antiarrhythmic drugs.

Principles of safety pharmacology.

Safety Pharmacology is a rapidly developing discipline that uses the basic principles of pharmacology in a regulatory-driven process to generate data to inform risk/benefit assessment. The aim of Safety Pharmacology is to characterize the pharmacodynamic/pharmacokinetic (PK/PD) relationship of a drug's adverse effects using continuously evolving methodology. Unlike toxicology, Safety Pharmacology includes within its remit a regulatory requirement to predict the risk of rare lethal events. This gives Safety Pharmacology its unique character. The key issues for Safety Pharmacology are detection of an adverse effect liability, projection of the data into safety margin calculation and finally clinical safety monitoring. This article sets out to explain the drivers for Safety Pharmacology so that the wider pharmacology community is better placed to understand the discipline. It concludes with a summary of principles that may help inform future resolution of unmet needs (especially establishing model validation for accurate risk assessment). Subsequent articles in this issue of the journal address specific aspects of Safety Pharmacology to explore the issues of model choice, the burden of proof and to highlight areas of intensive activity (such as testing for drug-induced rare event liability, and the challenge of testing the safety of so-called biologics (antibodies, gene therapy and so on.).

Pharmacological and toxicological activity of RSD921, a novel sodium channel blocker.

BACKGROUND: RSD921, the R,R enantiomer of the kappa (k) agonist PD117,302, lacks significant activity on opioid receptors. METHODS: The pharmacological and toxicological actions were studied with reference to cardiovascular, cardiac, antiarrhythmic, toxic and local anaesthetic activity. RESULTS: In rats, dogs and baboons, RSD921 dose-dependently reduced blood pressure and heart rate. In a manner consistent with sodium channel blockade it prolonged the PR and QRS intervals of the ECG. Furthermore, in rats and NHP, RSD921 increased the threshold currents for induction of extra-systoles and ventricular fibrillation (VFt), and prolonged effective refractory period (ERP). In rats, RSD921 was protective against arrhythmias induced by electrical stimulation and coronary artery occlusion. Application of RSD921 to voltage-clamped rat cardiac myocytes blocked sodium currents. RSD921 also blocked transient (ito) and sustained (IKsus) outward potassium currents, albeit with reduced potency relative to sodium current blockade. Sodium channel blockade due to RSD921 in myocytes and isolated hearts was enhanced under ischaemic conditions (low pH and high extracellular potassium concentration). When tested on the cardiac, neuronal and skeletal muscle forms of sodium channels expressed in Xenopus laevis oocytes, RSD921 produced equipotent tonic block of sodium currents, enhanced channel block at reduced pH (6.4) and marked use-dependent block of the cardiac isoform. RSD921 had limited but quantifiable effects in subacute toxicology studies in rats and dogs. Pharmacokinetic analyses were performed in baboons. Plasma concentrations producing cardiac actions in vivo after intravenous administration of RSD921 were similar to the concentrations effective in the in vitro assays utilized. CONCLUSIONS: RSD921 primarily blocks sodium currents, and possesses antiarrhythmic and local anaesthetic activity.

An overview of the safety pharmacology society strategic plan.

Safety Pharmacology studies are conducted to characterize the confidence by which biologically active new chemical entities (NCE) may be anticipated as safe. Non-clinical safety pharmacology studies aim to detect and characterize potentially undesirable pharmacodynamic activities using an array of in silico, in vitro and in vivo animal models. While a broad spectrum of methodological innovation and advancement of the science occurs within the Safety Pharmacology Society, the society also focuses on partnerships with health authorities and technology providers and facilitates interaction with organizations of common interest such as pharmacology, physiology, neuroscience, cardiology and toxicology. Education remains a primary emphasis for the society through content derived from regional and annual meetings, webinars and publication of its works it seeks to inform the general scientific and regulatory community. In considering the future of safety pharmacology the society has developed a strategy to successfully navigate forward and not be mired in stagnation of the discipline. Strategy can be defined in numerous ways but generally involves establishing and setting goals, determining what actions are needed to achieve those goals, and mobilizing resources within the society to accomplish the actions. The discipline remains in rapid evolution and its coverage is certain to expand to provide better guidance for more systems in the next few years. This overview from the Safety Pharmacology Society will outline the strategic plan from 2016 to 2018 and beyond and provide insight into the future of the discipline which builds upon a previous strategic plan established in 2009.

Molecular characterisation of the interactions between olmesartan and telmisartan and the human angiotensin II AT1 receptor.

BACKGROUND AND PURPOSE: Whereas some angiotensin II (Ang II) type 1 receptor blockers (ARBs) produce surmountable antagonism of AT(1) receptors, others such as olmesartan and telmisartan display varying degrees of insurmountability. This study compared the molecular interactions of olmesartan and telmisartan with the human AT(1) receptor, using well characterised in vitro methods and model systems. EXPERIMENTAL APPROACH: CHO-K1 cells that stably express human AT(1) receptors (CHO-hAT(1) cells) were used in several pharmacological studies of olmesartan and telmisartan, including direct radioligand binding and inhibition of Ang II-induced inositol phosphate (IP) accumulation. KEY RESULTS: Both ARBs were found to be competitive antagonists that displayed high affinity, slow dissociation, and a high degree of insurmountability for the AT(1) receptor (the latter greater with olmesartan). Their receptor interactions could be described by a two-step process with the initial formation of a loose complex (IR) and subsequent transformation into a tight binding complex (IR*). In washout experiments, [(3)H] telmisartan dissociated from the receptor with a half-life of 29 min and the Ang II-mediated IP accumulation response was 50% maximally restored within 24 min, whereas values for [(3)H] olmesartan were 72 min and 76 min, respectively. CONCLUSIONS AND IMPLICATIONS: The high degree of insurmountability, slow dissociation, and high affinity of olmesartan for its receptor may relate to its ability to stabilise IR* via the carboxyl group of its imidazole core. In comparison, telmisartan displays a less potent interaction with the receptor.

An overview of the safety pharmacology career of Dr. C.R. Hassler.

Each year the Safety Pharmacology Society (SPS) recognizes an investigator who has had a marked impact upon the discipline. The 2016 recipient of the SPS Distinguished Service Award (DSA) was Dr. Craig R. Hassler. Dr. Hassler is one of the founding members of the SPS and has been actively engaged in physiological research for over 46years. Dr. Hassler delivered a talk entitled "My 43Years at Battelle Memorial Institute" to meeting attendees. In this article an overview is provided of the illustrious career of Dr. Hassler along with an account of the numerous animal models that were developed at Battelle under his guidance over the years.

Citrulline as a Biomarker for Gastrointestinal-Acute Radiation Syndrome: Species Differences and Experimental Condition Effects.

Animal models of hematopoietic and gastrointestinal acute radiation syndromes (ARS) have been characterized to develop medical countermeasures. Acute radiation-induced decrease of intestinal absorptive function has been correlated to a decrease in the number of intestinal crypt cells resulting from apoptosis and enterocyte mass reduction. Citrulline, a noncoded amino acid, is produced almost exclusively by the enterocytes of the small intestine. Citrullinemia has been identified as a simple, sensitive and suitable biomarker for radiation-induced injury associated with gastrointestinal ARS (GI-ARS). Here we discuss the effect of radiation on plasma citrulline levels in three different species, C57BL/6 mice, Göttingen minipigs and rhesus nonhuman primates (NHPs), measured by liquid chromatography tandem mass spectrometry (LC-MS/MS). The effects of experimental study conditions such as feeding and anesthesia were also examined on plasma citrulline levels in the NHPs. Both the mice and Göttingen minipigs were partial-body irradiated (PBI) with doses from 13-17 Gy and 8-16 Gy, respectively, whereas NHPs were total-body irradiated (TBI) with doses from 6.72-13 Gy. Blood samples were taken at different time points and plasma citrulline levels were measured in the three species at baseline and after irradiation. Basal plasma citrulline concentrations (mean ± SEM) in mice and minipigs were 57.8 ± 2.8 µM and 63.1 ± 2.1 µM, respectively. NHPs showed a basal plasma citrulline concentration of 32.6 ± 0.7 µM, very similar to that of humans (∼40 µM). Plasma citrulline progressively decreased after irradiation, reaching nadir values between day 3.5 and 7. The onset of citrulline recovery was observed earlier at lower radiation doses, while only partial citrulline recovery was noted at higher radiation doses in minipigs and NHPs, complete recovery was noted in mice at all doses. Plasma citrulline levels in NHPs anesthetized with ketamine and acepromazine significantly decreased by 35.5% (P = 0.0017), compared to unanesthetized NHPs. In the postprandial state, citrulline concentrations in NHPs were slightly but significantly decreased by 12.2% (P = 0.0287). These results suggest that plasma citrulline is affected by experimental conditions such as anesthesia and feeding.

An analysis of cardiac sodium channel properties using digital signal processing techniques.

The properties of single sodium channels in membrane patches from isolated rat ventricular myocytes were analyzed. Both cell-attached and inside-out patch configurations were examined. A digital signal processing method was used which allowed accurate determination of single-channel conductance and mean open time, even in situations where many channels coexisted in the patch. The results show that the cardiac sodium channel has a conductance of 5.3 pS at room temperature, and shows no rectification or saturation in the physiological ranges of ion concentration and membrane potential. The active channels contained in the membrane patch opened and closed independently of each other. A transition probability matrix, which describes transitions between all current levels, can also be generated, analysis of which permits an estimate of individual mean channel open time as well as the degree of coupling between channels.

Endogenous chemical mediators of ventricular arrhythmias in ischaemic heart disease.

The causes of ventricular arrhythmias in the acute setting of coronary artery disease (myocardial ischaemia and reperfusion) may be approached using two paradigms. One, the electrophysiological paradigm (disturbance of ionic homeostasis, electrogenesis, and conduction) has not been addressed in detail here. Instead, we have focused on the concept of a chemical paradigm of arrhythmogenesis. Many endogenous chemical substances (derived from the myocardium, nerves, blood plasma, platelets, leucocytes, and endothelium) accumulate in the ischaemic tissue or are produced during reperfusion and many of these have been suggested to modulate ventricular arrhythmias. Some substances may be arrhythmogenic and others may be antiarrhythmic. Together they determine whether or not arrhythmias occur. Potentially arrhythmogenic substances include potassium, catecholamines, cAMP, histamine, 5-HT, lysophosphatidylcholine, palmitylcarnitine, platelet activating factor, prostaglandins, leukotrienes, thromboxane A2, angiotensin II, endothelin, opioids, protons, calcium, and free radicals. We have considered each of these, with the objective of evaluating which are important in arrhythmogenesis in acute ischaemia and reperfusion. Two alternative models of arrhythmogenesis are possible in the context of the chemical paradigm: a series model (where one substance or its effects determines the arrhythmogenicity of another) and a parallel model (where numerous substances operate independently to cause ventricular arrhythmias). It is not yet clear which model is most appropriate; a combination of the two is possible, so a working prototype has been constructed which accommodates both. A set of criteria (hitherto lacking) for establishing whether a substance is sufficient and necessary for arrhythmogenesis is proposed. Some generalisations are given on approaches to establishment of these criteria for putative arrhythmogenic substances. Finally, we have considered how arrhythmogenic drug development may be influenced by using the chemical paradigm as an alternative to the electrophysiological paradigm of arrhythmogenesis.

Cardiac transplantation does not effect ischaemia-induced arrhythmias in rats.

OBJECTIVE: In many species arrhythmias induced by myocardial ischaemia appear to be in part dependent upon cardiac sympathetic nerves. However, previous experiments in rats did not suggest that myocardial or other catecholamines are involved in ischaemic arrhythmogenesis in this species. The aim of this study was to investigate this further using transplanted hearts. METHODS: We transplanted 'donated' hearts onto the abdominal aorta of recipient rats and, at varying periods after transplantation, subjected donated and recipient hearts to occlusion of the left anterior descending (LAD) coronary artery. Donated and recipient hearts were tested at various times after transplantation for responsiveness to drugs acting upon aspects of the autonomic nervous system. The intention of this latter study was to assess the status of innervation and receptors simultaneously in both donated and recipient hearts. RESULTS: Donated (transplanted) hearts showed responses consistent with denervation and receptor supersensitivity. Changes varied with the duration of the transplant. Over the same period recipient hearts did not change in responsiveness to drugs. When subjected to coronary artery occlusion, transplanted hearts responded to occlusion with the same frequency and severity of arrhythmias as recipient and other control hearts, regardless of the duration of transplant, or sensitivity to drugs. CONCLUSIONS: The results of these experiments suggest that cardiac innervation is not an important factor in the genesis of ischaemia-induced arrhythmias in rats.

Tonic and use-dependent block of sodium currents in isolated cardiac myocytes by bisaramil.

1. The effects of bisaramil on sodium currents in rat isolated cardiac myocytes were examined by use of tight-seal, whole-cell patch clamp techniques. Bisaramil produced a concentration-dependent, readily reversible reduction in peak transient sodium current. When the sodium current was evoked at 3 s intervals the estimated ED50 for bisaramil was about 11 microM. 2. Bisaramil (16 microM) produced a shift in the inactivation curve to hyperpolarized potentials of about 10 mV, but produced no change in the voltage-dependence of activation. 3. The block of the sodium current by bisaramil showed a profound use-dependence. A concentration of 10 microM produced a considerable block of the current with repeated stimulation. The recovery from block was biphasic, showing fast and slow components which had time constants of about 40 ms and 5 s respectively. 4. Bisaramil produced little tonic block of the sodium current at concentrations of 100 microM; at 300 microM it produced tonic block of around 50%, with extreme use-dependence. 5. Bisaramil appeared not to interact primarily with the inactivated form of the channel, since lengthening the depolarizing pulses did not affect the degree of block produced.

Molecular analysis of the Na+ channel blocking actions of the novel class I anti-arrhythmic agent RSD 921.

RSD 921 is a novel, structurally unique, class I Na+ channel blocking drug under development as a local anaesthetic agent and possibly for the treatment of cardiac arrhythmias. The effects of RSD 921 on wild-type heart, skeletal muscle, neuronal and non-inactivating IFMQ3 mutant neuronal Na+ channels expressed in Xenopus laevis oocytes were examined using a two-electrode voltage clamp. RSD 921 produced similarly potent tonic block of all three wild-type channel isoforms, with EC50 values between 35 and 47 microM, whereas the EC50 for block of the IFMQ3 mutant channel was 110+5.5 microM. Block of Na+ channels by RSD 921 was concentration and use-dependent, with marked frequency-dependent block of heart channels and mild frequency-dependent block of skeletal muscle, wild-type neuronal and IFMQ3 mutant channels. RSD 921 produced a minimal hyperpolarizing shift in the steady-state voltage-dependence of inactivation of all three wild-type channel isoforms. Open channel block of the IFMQ3 mutant channel was best fit with a first order blocking scheme with k(on) equal to 0.11+/-0.012x10(6) M(-1) s(-1) and k(off) equal to 12.5+/-2.5 s(-1), resulting in KD of 117+/-31 microM. Recovery from open channel block occurred with a time constant of 14+/-2.7 s(-1). These results suggest that RSD 921 preferentially interacts with the open state of the Na+ channel, and that the drug may produce potent local anaesthetic or anti-arrhythmic action under conditions of shortened action potentials, such as during anoxia or ischaemia.

Electrophysiological and antiarrhythmic actions of the kappa agonist PD 129290, and its R,R (+)-enantiomer, PD 129289.

1. The S,S (-)-enantiomer PD 129290, a kappa agonist, and its corresponding inactive R,R (+)-enantiomer (PD 129289) were studied in rat isolated hearts and in intact rats for cardiovascular and antiarrhythmic actions. The electrophysiological actions of PD 129290 were also studied in rat isolated cardiac myocytes using voltage clamp. 2. Ventricular pressure, heart rate and ECG were studied in isolated hearts while blood pressure, heart rate and ECG were studied in pentobarbitone-anaesthetized rats. In the latter, responses to electrical stimulation and coronary occlusion were also investigated. 3. In isolated hearts both enantiomers, over the concentration range 2-16 microM, dose-dependently reduced systolic ventricular pressure and heart rate while prolonging the P-R and QRS intervals of the ECG. 4. At doses of 1-32 mumol kg-1 both enantiomers reduced blood pressure and heart rate in anaesthetized rats. In addition, both enantiomers increased the size of the RSh and increased P-R, QRS, and Q-T intervals of the ECG. The thresholds for premature beats and ventricular fibrillation were dose-dependently increased by PD 129289. At lower doses PD 129290 decreased thresholds. These decreases were blocked by naloxone to reveal underlying increases similar to those seen with PD 129289. Both enantiomers increased refractory periods. 5. Naloxone (8 mumol kg-1) did not alter any of the actions of PD 129290, except to abolish the initial decreases in thresholds in intact rats seen with lower doses of PD 129290. 6. Both enantiomers (2 and 8 mumol kg-1) equally reduced arrhythmias in anaesthetized rats subject to occlusion of a coronary artery. 7. In rat isolated cardiac myocytes 20 microM PD 129290, in the presence and absence of naloxone decreased the amplitude of the transient sodium current by about 50% without affecting the voltage dependence of activation or inactivation of this current.8. The antiarrhythmic actions of both enantiomers appear to primarily result from their Class I(sodium channel blockade) properties which are independent of kappa agonism.

Cardiovascular actions of the kappa-agonist, U-50,488H, in the absence and presence of opioid receptor blockade.

1. The cardiovascular actions of U-50,488H, a kappa-receptor agonist, were studied in rat isolated perfused hearts, and in anaesthetized rats, over concentrations or doses generally above those required to produce kappa-receptor-mediated effects. 2. U-50,488H dose-dependently decreased left-ventricular peak systolic pressure and beating rate in vitro and reduced blood pressure and heart rate in vivo. 3. Over the concentration range of 1-30 microM in vitro, and the dose-range of 0.5-32 mumol kg-1 in vivo, U-50,488H prolonged the P-R, QRS and Q-T intervals of the ECG. 4. The effects of U-50,488H were not antagonized by an opioid receptor antagonist, naloxone (1 microM or 8 mumol kg-1). Similarly, the opioid receptor antagonist, MR 2266, at 8 mumol kg-1 did not significantly reduce the cardiovascular actions of U-50,488H in vivo. 5. The actions of U-50,488H on responses to electrical stimulation were also studied. Over the dose range of 0.5-32 mumol kg-1, U-50,488H altered thresholds and effective refractory period. It had a biphasic action on thresholds for induction of ventricular fibrillation. Thresholds were decreased at lower doses (0.5-4 mumol kg-1) but increased at higher doses (8-32 mumol kg-1). The effects of lower doses were blocked by naloxone. Effective refractory period and threshold pulse width only increased with dose. 6. In conclusion, U-50,488H at high concentration, had direct depressant actions on cardiac contractility, electrical excitability and the ECG. These depressant effects were not antagonized by the opioid receptor antagonists, naloxone and MR 2266, and probably do not involve opioid receptors. Furthermore, some of the observed effects were those expected to result from sodium channel blockade.

Etanercept. Immunex.

Immunex has developed and launched etanercept, a soluble TNF receptor (TNFR) fusion protein, for the treatment of rheumatoid arthritis (RA). It has also been developed for various TNF-mediated conditions such as congestive heart failure, endometriosis and multiple sclerosis. Etanercept has been launched as a second-line agent in the US for the treatment of moderate-to-severe RA and can be used in conjunction with methotrexate in patients unresponsive to methotrexate alone. It is also available in the EU. In 2000, it was in phase III trials for psoriatic arthritis and an NDA filing for this indication was expected for the first half of 2001. In July 2001, the sBLA was filed, and in September 2001, the FDA granted the sBLA Priority Review status. As of January 2001, etanercept was in phase III trials for congestive heart failure, with sNDA filing expected in 2002; however, by March 2001, these had been halted, as it did not appear that statistical significance would be reached for the efficacy endpoints. Further data analysis was being undertaken at this time, before a final decision was taken. In April 2001, Merrill Lynch reported that development for this indication was to be halted. Sales for the drugs first full quarter on the market in 1999 were US $59.7 million. By November 1999 the drug had made sales of US $500 million; Immunex expected the drug to generate over US $2 billion in annual sales by 2004. In September 2000, Merrill Lynch reported that if sales of the drug continued at the present rate then it is likely that demand would temporarily outstrip supply in 2001. Resolution of the supply issue was expected by 2002. Also in September 2000, Merrill Lynch lowered their estimate of sales in 2001 from US $1 billion to $927 million. In the long-term, Merrill Lynch believed that the drug has the potential to exceed US $5 billion in sales in the US. In April 2001, Merrill Lynch predicted that etanercept prescribed for RA would generate sales of US $71 in 2002 rising to US $600 million in 2005. In October 2001, Morgani Stanley reported that Enbrel continues to be the primary source of revenue of Immunex (US $198.1 million). It was also reported that if launched for CHF, an estimated peak year revenue was likely to be US $500 million. The company maintains a website containing additional information about etanercept at http://www.enbrelinfo.com.

Effects of bisaramil, a novel class I antiarrhythmic agent, on heart, skeletal muscle and brain Na+ channels.

The effects of bisaramil, a novel diazabicyclononane antiarrhythmic agent, were compared to those of lidocaine, a clinically used class Ib antiarrhythmic agent, on heart, skeletal muscle and brain Na+ channels expressed in Xenopus laevis oocytes using a two-electrode voltage clamp. Both bisaramil and lidocaine produced a concentration-dependent tonic block of Na+ current that was most effective on cardiac channels, but bisaramil was more potent than lidocaine. Both drugs produced a concentration-dependent shift in the voltage-dependence of inactivation and delayed recovery from inactivation. Bisaramil produced marked frequency-dependent block of heart channels and mild frequency-dependent block of skeletal muscle and brain channels, whereas lidocaine produced marked frequency-dependent block of all three channel types. Therefore, bisaramil shows tonic and frequency-dependent blockade that is most potent against the heart Na+ channel, which may account for its potent antiarrhythmic efficacy in vivo, and may result in reduced central nervous system toxicity compared to clinically used agents such as lidocaine.

An electrophysiological basis for the antiarrhythmic actions of the kappa-opioid receptor agonist U-50,488H.

This study examined the actions of the selective kappa-opioid receptor agonist, U-50,488H, on voltage activated Na+ and K+ currents in isolated rat cardiac myocytes. U-50,488H produced a concentration-dependent block of the transient Na+ current with an ED50 of about 15 microM, and, at higher concentrations (40-50 microM), a block of the plateau K+ current and an increase in the rate of decay of the transient K+ current. In addition U-50,488H produced a hyperpolarising shift in the inactivation curve for the transient Na+ current without altering the voltage dependence for activation and without an effect on the voltage dependence of inactivation or activation of K+ currents. The block of Na+ currents by U-50,488H showed pronounced use dependence. The kappa-opioid receptor antagonist MR2266 did not itself produce any change in the Na+ or K+ currents and did not change the channel blocking properties of U-50,488H. Thus, since the antiarrhythmic actions of U-50,488H are not blocked by MR2266 or naloxone, the effects of U-50,488H to block Na+ and K+ currents are the most likely reasons for its antiarrhythmic actions, rather than an action at kappa-opioid receptors.

Antiarrhythmic effects of U-50,488H in rats subject to coronary artery occlusion.

The antiarrhythmic actions of low and high doses of U-50,488H, a selective kappa-receptor agonist, were examined in pentobarbitone-anaesthetized rats subjected to occlusion of the left anterior descending coronary artery. At a high dose (16 mumol/kg) U-50,488H reduced blood pressure, heart rate and prolonged the P-R and QRS intervals of the electrocardiogram. This dose reduced the incidence of ventricular arrhythmias produced by occlusion. The blood pressure, heart rate, ECG and antiarrhythmic actions of a high dose of U-50,488H were not antagonized by 8 mumol/kg naloxone, a dose which had no cardiovascular or ECG actions. Naloxone alone reduced arrhythmia incidence but to a lesser extent than U-50,488H. A low dose (0.2 mumol/kg) of U50,488H in the absence or presence of naloxone had no effect on arrhythmias. Thus, U-50,488H had antiarrhythmic actions at a high dose which were independent of opioid receptors.

The cardiac electrophysiological effects of sparteine and its analogue BRB-I-28 in the rat.

This study compares the cardiovascular and antiarrhythmic effects of sparteine and a 3,7-diheterobicyclo[3.3.1]nonane analogue of sparteine, BRB-I-28, in pentobarbitone-anaesthetized rats subjected to left-ventricle electrical stimulation and occlusion of the left anterior descending coronary artery. Sparteine and BRB-I-28 produced a dose-dependent reduction in heart rate and blood pressure over the dose range 1-64 mumol/kg/min. As well, the P-R and Q-aT intervals of the electrocardiogram (ECG) were prolonged. The thresholds for induction of premature beats and ventricular fibrillation were dose-dependently increased and both drugs increased refractoriness. While sparteine and BRB-I-28 (at 16 and 64 mumol/kg/min, respectively) did not change the incidence of premature beats or ventricular tachycardia with coronary occlusion, both drugs equally reduced the incidence of ventricular fibrillation. We characterized the actions of sparteine and BRB-I-28 on cardiac Na+, transient outward and sustained outward plateau K+ currents of rat myocytes using the whole-cell patch-clamp. Sparteine and BRB-I-28 produced a concentration-dependent reduction in Na+ current with EC50 values of 110 and 230 microM, respectively. Both drugs produced hyperpolarizing shifts of 8 and 11 mV, respectively, for Na+ channel inactivation while neither produced a change in channel activation. Both drugs produced a concentration-dependent block of the sustained plateau K+ current and increased the rate of decay of the transient outward K+ current. Thus, sparteine and BRB-I-28 possess Na+ and K+ channel blocking properties which may account for their antiarrhythmic actions against electrical and ischaemic arrhythmias.